Yeah, that's were I was going with this...
While having a problem with two cyliners right next to each other might seem like a fuel delivery problem, it may just be a coincidence, and that the problem is really something else.
I would suggest doing a compression test on it. I wouldn't be surprised if the middle two were low, most likely due to bad rings, which is causing oil consumption in those two cylinders.
The only thing I was thinking about if it was the carb, is that its dumping fuel after shutoff, and that the grooves in the bottom of the intake were somehow directing most of it to the middle port. It's a long shot...
As for the PCV, another long shot, but if it was hooked into the middle runner,like the factory did it, and you had alot of blowby, you may just be adding too much oil vapor to those cylinders for the spark to overcome.
Or you don't have enough spark at those two cylinders due to a problem with the the cap or plug wires.
Do a compression test first...
